diff --git a/tests/adapter/matrix/test_files.py b/tests/adapter/matrix/test_files.py
index 831ca72..71fb02f 100644
--- a/tests/adapter/matrix/test_files.py
+++ b/tests/adapter/matrix/test_files.py
@@ -48,3 +48,20 @@ async def test_download_matrix_attachment_persists_file_and_returns_workspace_pa
     assert saved.workspace_path is not None
     assert saved.workspace_path.endswith("20260420-153000-report.pdf")
     assert (tmp_path / saved.workspace_path).read_bytes() == b"%PDF-1.7"
+
+
+def test_build_workspace_attachment_path_keeps_room_safe_agents_relative_contract(tmp_path: Path):
+    rel_path, abs_path = build_workspace_attachment_path(
+        workspace_root=tmp_path / "agents" / "7",
+        matrix_user_id="@alice+bob:example.org",
+        room_id="!room/ops:example.org",
+        filename="quarterly status (final).pdf",
+        timestamp="20260420-153000",
+    )
+
+    assert rel_path == (
+        "surfaces/matrix/alice_bob_example.org/room_ops_example.org/inbox/"
+        "20260420-153000-quarterly_status_final_.pdf"
+    )
+    assert not Path(rel_path).is_absolute()
+    assert abs_path == tmp_path / "agents" / "7" / rel_path
diff --git a/tests/platform/test_real.py b/tests/platform/test_real.py
index 7a2e37e..81a73b2 100644
--- a/tests/platform/test_real.py
+++ b/tests/platform/test_real.py
@@ -190,6 +190,7 @@ async def test_real_platform_client_forwards_attachments_to_chat_api():
     agent_api = FakeAgentApiFactory(chat_api_cls=AttachmentTrackingChatAgentApi)
     client = make_real_platform_client(agent_api)
     attachment = Attachment(
+        url="/agents/7/surfaces/matrix/alice/room/inbox/report.pdf",
         workspace_path="surfaces/matrix/alice/room/inbox/report.pdf",
         mime_type="application/pdf",
         filename="report.pdf",
@@ -210,6 +211,20 @@ async def test_real_platform_client_forwards_attachments_to_chat_api():
     assert result.tokens_used == 0
 
 
+def test_attachment_paths_normalize_workspace_roots_to_relative_paths():
+    attachments = [
+        Attachment(workspace_path="/workspace/output/report.pdf"),
+        Attachment(workspace_path="/agents/7/output/report.csv"),
+        Attachment(workspace_path="surfaces/matrix/alice/room/inbox/note.txt"),
+    ]
+
+    assert RealPlatformClient._attachment_paths(attachments) == [
+        "output/report.pdf",
+        "output/report.csv",
+        "surfaces/matrix/alice/room/inbox/note.txt",
+    ]
+
+
 @pytest.mark.asyncio
 async def test_real_platform_client_preserves_send_file_events_in_sync_result(monkeypatch):
     class FileEventAgentApi(AttachmentTrackingChatAgentApi):
@@ -239,6 +254,26 @@ async def test_real_platform_client_preserves_send_file_events_in_sync_result(mo
     ]
 
 
+@pytest.mark.parametrize(
+    ("location", "expected_workspace_path"),
+    [
+        ("/workspace/output/report.pdf", "output/report.pdf"),
+        ("/agents/7/output/report.pdf", "output/report.pdf"),
+        ("surfaces/matrix/alice/room/inbox/report.pdf", "surfaces/matrix/alice/room/inbox/report.pdf"),
+    ],
+)
+def test_attachment_from_send_file_event_normalizes_shared_volume_paths(
+    location: str, expected_workspace_path: str
+):
+    attachment = RealPlatformClient._attachment_from_send_file_event(
+        MsgEventSendFile(path=location)
+    )
+
+    assert attachment.url == location
+    assert attachment.workspace_path == expected_workspace_path
+    assert attachment.filename == "report.pdf"
+
+
 @pytest.mark.asyncio
 async def test_real_platform_client_uses_fresh_agent_connection_per_request():
     agent_api = FakeAgentApiFactory()